Purpose To describe the sonographic (ultrasound—US) features of retained surgical sponges (RSSs) and compare them with the pathological findings. Methods Ultrasound features of RSSs in nine patients (seven women and two men) identified between June 1996 and July 2015 were retrospectively analyzed. Patient characteristics including gender and age, location of the sponge, time interval until diagnosis, clinical presentation, and patient complaints were evaluated. Results The US appearances of RSSs could be classified into three types. Type I (five cases): an echogenic arc with a strong posterior shadow; type II (two cases): US appearance mimicked a cystic teratoma; type III (two cases): a cystic mass with zigzag‐shaped internal contents. CT and/or MRI showed a mass with density/signal intensity similar to that of the adjacent soft tissues. Conclusion The characteristic US findings along with a history of surgery can help reach a correct diagnosis of RSS.